Geralt the Witcher, Hogwarts Professor

Geralt stepped out of his office and found his classroom full of fifteen- and sixteen-year-old children. Each one wore black Hogwarts robes and sat attentively. Their chatter died out as soon as he appeared.

They'd heard of him.

All right; he didn't have to introduce himself.

"I have one lesson for you," he began. "I will teach it to you every class, and maybe, after the school year is done, you might develop a vague understanding of it."

A boy's hand shot into the air.

"Yes?" Geralt asked.

"Professor Rivia, will this 'one lesson' cover everything that will be on our Ordinary Wizarding Level examination?"

"It will set you up for every form of success that matters," Geralt said. He picked up a piece of chalk and wrote a single word on the board: WHY.

He looked at the children's blank expressions. Then he underlined the word and circled it for emphasis.

"The key to defending oneself against the Dark Arts, or any dangerous beast in the world, or even any person; is understanding the motivation behind the harm."

A girl raised her hand.

"Yes?"

"Sir, will you be teaching us counterjinxes? My brother said his O.W.L. had a practical exam with a lot of counterjinxes."

"Yes, yes," Geralt said with a dismissive wave of his hand. "You'll learn everything you need to pass your examination, and, more importantly, to defend yourself against the dark arts."

Three more students raised their hands.

"I'm going to start docking points from people who ask me about O.W.L.'s," Geralt said.

Two of the students lowered their hands. The first boy kept his up.

"Yes?"

"Sir, are you going to take roll call? Or pass out a class syllabus?"

"No roll call," Geralt said. "If you want to learn how to survive in a variety of dangerous situations, you'll come to class. If you skip my lessons, you’ll risk more than writing lines in detention. And as for a syllabus."

He returned to the chalkboard and updated the message to read, "Find out WHY."

"This is it," Geralt said. "This is the syllabus. Your one lesson. Your key to self-defense. Find out why you are in danger, and you can mitigate the danger. Question: why does an acromantula attack a person? Don't bother raising hands, just let me hear it."

"The spider is hungry," a student said.

"Or the person invaded the spider's web," another added.

"The acromantula could be afraid of attack," another commented.

Geralt nodded. "Anyone else?"

A girl with a blue and bronze prefect badge ventured to say, "Maybe the acromantula was befriended by someone cruel, and attacks to support their friend."

"Well done," Geralt told her. "Five points to... Ravenclaw house, right?"

"Yes, thank you," the girl said.

Geralt continued: "You know acromantulas have huge appetites, so you can avoid danger by ensuring they have easier prey than you. Steer clear of their webs. Behave in non-threatening ways around them. And if all else fails, arania exumai repels spiders."

A girl with a yellow and black necktie asked, "Will arania exumai be on the O.W.L.?"

"Two points from Hufflepuff," Geralt said. "I don't write the Ordinary Wizarding Level examinations. I will teach you everything I can, to prepare you for your test and real life. But it will always, always come back to:" he tapped the word on the board, "Why."

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Based on a prompt by u/Adventurous-Art-1161 on r/WritingPrompts: “[WP] The Hogwarts students chatted anxiously among themselves in excitement. They had heard a lot about the new deputy instructor of ‘Defense against the Dark Arts’ - Geralt of Rivia, the killer of the king.”